"It was the best race I could pull off at 9:30 in the morning. I'm pretty glad with that," said Fratus.

Sarah Sjostrom looked in commanding form as the super Swede advanced to the women’s 50m fly semi-finals, recording 25.25 seconds to take top spot in qualification.

Sjostrom is overwhelming favourite to land gold in the women's 100m freestyle final later as she attempts to break a milestone barrier for the second time at these championships.

"I will aim to go under 52 seconds tonight but I think it will be impossible to break the world record again," said Sjostrom.

Kathleen Baker of the United States qualified fastest ahead of local hope Katinka Hosszu in the women's 200m backstroke semis.

Baker's compatriot Katie Ledecky had plenty of energy to spare in the women's 800m freestyle preliminaries as the indefatigable American recorded eight minutes 20.24. Leah Smith of the U.S. was second fastest, 0.55 seconds back.